It was said that the aliens would not accept gamete reproduction because they ate children before they knew what evolution was. They did it because it increased the number of surviving children from each pen/ couple. If they are still culling to increase the success rate, than a new method that increased their success rate would be preferable. In the story, gametes are better for evolution than culling so gametes would produce more surviving off-spring and would thus be preferable to culling.
I said "in the story" gametes are better because I disagree with the author on which is better for evolution. Producing good gametes will allow one to get an off-spring instead of rival mates, but the best gamete doesn't necessarily make the best off-spring. Gametes are selected by which one is the best swimmer and which has the best 'sense of smell'. It could be coded to produce a child with extremely low intelligence or extremely poor muscle tone, or unable to get glycogen out of their cells (Pompe). If out of the millions of sperm emitted with each ejaculation, the best tended to win, there would be much lower rates of genetic disease.
They said that the alien would expect that if Hitler were bad he wouldn't have been made a leader or would have been killed. Most of the world disliked genocide, but the Nazi, being loyal, liked what ever Germany was doing so they liked genocide. This group was wrong and is the reason Hitler was the leader. I'm not saying the author messed up due to this ; its just something I noticed. A potential flaw was it was said if Hitler had been bad humans would have killed him. We did try to kill him and then tried the people who protected him for war crimes, although the Nazis being a group, it might have been seen as a 'reasonable error" esp. since most Nazis weren't killed.

See, that's why you don't say "because" about evolution. They eat children because-as-cognitive-motivation they have a drive to eat children. They have a drive to eat children because-as-evolutionary-history it increased the number of surviving children.